Blown film extrusion device

ABSTRACT

The invention describes a blown film extrusion device with a blow head ( 20 ) and an outer cooling ring ( 10 ) mountable on its top ( 21 ), which has an annular support surface ( 11 ) on its bottom and can be centered relative to blow head ( 10 ) via centering devices. 
     The centering devices are formed by at least three radially arranged linear guides, each of which has a radially extending recess ( 12.1, . . . , 12.4 ) and at least one guide element ( 22.1, . . . , 22.4 ) engaging in it.

The invention concerns a blown film extrusion device with a blow head and an external cooling ring mountable on its top, having an annular support surface on its bottom and centerable relative to the blow head via centering devices.

Such a device is known from DE 33 36 181 A1. The so-called blow head has an annular nozzle gap from which plastic melt emerges. Air is blown into the center of the annular nozzle. At the same time air is fed from the outside by means of an external cooling ring against the film tube emerging at the annular nozzle. The external cooling ring is then mounted with an annular surface on its bottom on an annular surface on the top of the blow head so that the external cooling ring can be slightly raised in order to facilitate access to the annular nozzle during maintenance. A problem here is that the already slight eccentricities or oblique positions of the mounted external cooling ring mean that the bubble taken off from the annular nozzle is slanted, which can lead to nonuniform thickness distribution in the film and to problems during further processing.

The centering devices proposed in the known blown film extrusion device are screws, which are aligned on the bottom of the external cooling ring in the radial direction and mounted in a threaded hole. They can be set with their tip against the outer periphery of a circular section in the upper area of the blow head.

Centering occurs in a blown film extrusion device according to DE 92 14 647 U1 through a shoulder with the shape of a truncated cone on the blow head, on which the outer cooling ring is mounted with a conical hole.

It is also known to form centering devices by hook-like angles. The upper edge of the blow head can be enclosed by at least three angles on the bottom of the outer cooling ring so that the centering ring is centered relative to the blow head.

All known blown film extrusion devices have the common feature that their centering devices do not permit temperature-independent centering. Centering of the cooling ring and blow head free of play performed at room temperature is not effective in operation, since the stronger heating of the blow head leads to relatively greater elongation than in the cooling ring. Significant pressure forces would therefore act on the centering devices that are supported on the outer periphery of parts of the blow head. For this reason, centering must be conducted separately at each operating point in the known blown film extrusion devices.

The task of the invention is therefore to permit temperature-independent centering of the outer cooling ring and blow head.

This task is solved in a blown film die of the type just mentioned in that the centering devices are formed by at least three radially arranged linear guides, each of which includes a radically extending recess and at least one guide element engaging in it.

Because of the radial extent of the linear guides, a different temperature elongation of the two centering parts in the peripheral direction is possible relative to each other. The temperature elongation only leads to a relative shift of the guide elements within the groove in the radial direction.

Owing to the fact that at least three linear guides are formed, forced centering is present regardless of whether the guide elements are arranged on the bottom of the outer cooling ring or on the top of the blow head, so that both arrangements are possible.

A design with four grooves, two each lying on a common diameter line, i.e., flush with each other, simplifies manufacture and increases the accuracy.

To facilitate mounting of the cooling ring on the blow head, self-centering is caused by the fact that the feather key elements are beveled on their radially aligned side edges.

In addition or as an alternative, it is possible to bevel the grooves on their radially aligned side edges so that the feather key elements slide easily into the grooves when mounted.

Instead of feather key elements, pins with a circular cross section can also be provided as guide elements. The advantage of pins is that only linear contact on both flanks of the groove occurs and that heat transfer from the blow head into the corresponding guide element is therefore reduced.

In the blown film extrusion device according to the invention the groove and guide element can be designed with their side dimensions for loose fit, in which the fit is chosen so that even on maximal heating of the blow head during operation as well as simultaneous maximal cooling of the blow head, sliding of the guide elements within the groove is possible in each case and jamming is prevented.

To further improve accuracy of centering in all operating states the following measures are proposed:

-   -   There is a possibility of producing the guide elements from a         material that has the same heat expansion coefficient as the         housing of the joint partner in the area of the groove. At the         same time the guide elements are decoupled by thermal insulation         from the component to which they are applied. In this case a         rectangular shape of the guide element is advantageous, since         longer linear contact occurs and the guide element is heated to         roughly the same temperature as the groove area in which it         engages. Consequently, the groove and guide element expand the         same way at the corresponding engagement point so that a         correspondingly closer fit can be chosen and therefore higher         centering accuracy achieved.     -   Another possible measure to restrict lateral play consists of         configuring the groove either entirely V-shaped or milling out         the groove bottom V-shaped and at the same time giving the guide         elements a wedge shape on their tip. The reverse arrangement is         also possible, in which a V-shaped rib protrudes from the base         of the groove, onto which a compatible recess on the tip of the         guide element fits. In particular, if the wedge shapes do not         agree in angle, a sharply delimited linear contact is achieved         so that the radial movement of the guide element is possible         without jamming of the guide element in the groove or an         oversize in a reverse arrangement, which might lead to         inaccuracy.

The invention is further explained below with reference to the drawing.

The figures show in detail:

FIG. 1 shows the blown film extrusion device according to the invention in a perspective view,

FIG. 2 shows a top view of the bottom of the outer cooling ring and

FIG. 3 shows the blow head and outer cooling ring in a perspective view true to position.

FIG. 1 shows the necessary base elements of a blown film extrusion device without a cooling ring 10 and a blow head 20. The blow head is shown in the correct position, i.e., with top facing up, whereas the cooling ring 10 mountable on an annular surface 21 on the top of blow head 20 is rotated by 180°, i.e., positioned on the head in order to permit a view of the bottom and the centering elements arranged there.

Four guide elements in the form of feather keys 22.1, . . . , 22.4 are arranged on the upward facing and horizontally aligned annular surface 21 on blow head 20, which are aligned exactly radially, as indicated by the dash-dot diameter lines. The center points 13, 23 of the outer cooling ring 10 and blow head 20 to be centered relative to each other lie at the intersection point of the diameter lines.

A circular surface 11 is present on the bottom of the outer cooling ring 10, in which four grooves 12.1, . . . 12.4 are made, which extend precisely radially. Both the guide elements 22.1, . . . , 22.4 as well as the grooves 12.1, . . . , 12.4 are offset by exactly 90° relative to each other in the depicted practical example.

In order to be able to compensate for manufacturing tolerances, the feather keys 22.1, . . . , 22.4 are initially joined by a screw to the annular surface 21 and then, after exact alignment relative to grooves 12.1, . . . , 12.4 by alignment pins are finally fastened in their position relative to annular surface 21.

FIG. 2 shows the arrangement of grooves 12.1, . . . , 12.4 again in a top view on the bottom with its annular somewhat raised support surface 11. Opposite grooves 12.1, 12.3 and 12.2, 12.4 lie on a diameter line, in which the diameter lines are aligned offset by 90° relative to each other.

FIG. 3 schematically depicts the view from the outer periphery of an outer cooling ring 10 positioned on blow head 20 in the area of a linear guide. This is formed in the practical example depicted here again by a radially extending recess 12′ in the outer cooling ring 10 and by the guide elements 22′ protruding from the blow head top. The groove 12′ and the guide element 22′ are V-shaped or wedge-shaped. The enclosed angle in the convex parts, here the guide elements 22′ are smaller than in the concave parts, here the grooves 12′ so that linear contact only occurs in the area of the tip.

LIST OF REFERENCE NUMBERS

10 Outer cooling ring

11 Circular surface

12′ Groove

12.1 Groove

12.2 Groove

12.3 Groove

12.4 Groove

13 Centering center point

20 Blow head

21 Annular surface

22′ Guide element

22.1 Guide element

22.2 Guide element

22.3 Guide element

22.4 Guide element

23 Centering center point 

1. Blown film extrusion device with a blow head (20) and an outer cooling ring (10) mountable on its top (21), which has an annular support surface (11) on its bottom and can be centered relative to blow head (10) via centering devices, characterized by the fact that the centering devices are formed by at least three radially arranged linear guides, each of which includes a radially extending recess (12.1, . . . , 12.4) and at least one guide element (22.1, . . . , 22.4) engaging in it.
 2. Blown film extrusion device according to claim 1, characterized by the fact that the linear guides are formed by grooves (12.1, . . . , 12.4) on the bottom (11) of outer cooling ring (10) and guide elements (22.1, . . . , 22.4) engaging in it, which protrude from the top (21) of blow head (20).
 3. Blown film extrusion device according to claim 1, characterized by the fact that the linear guides are formed by grooves on the top of the blow head and guide elements engaging in it, which protrude from the bottom of the outer cooling ring.
 4. Blown film extrusion device according to claim 2, characterized by the fact that guide elements (22.1, . . . , 22.4) are formed by radially aligned feather key elements.
 5. Blown film extrusion device according to claim 4, characterized by the fact that the feather key elements (22.1, . . . , 22.4) are beveled on their radially aligned side edges.
 6. Blown film extrusion device according to claim 2, characterized by the fact that the guide elements are formed by pins with a circular cross section.
 7. Blown film extrusion device according to claim 2, characterized by the fact that the grooves (12.1, . . . , 12.4) are beveled on their radially aligned side edges.
 8. Blown film extrusion device according to claim 1, characterized by the fact that four grooves (12.1, . . . , 12.4) are provided, two each lying on a common diameter line.
 9. Blown film extrusion device according to claim 1, characterized by the fact that the guide elements (22.1, . . . , 22.4) are designed thermally decoupled.
 10. Blown film extrusion device according to claim 1, characterized by the fact that in a section across the radial extent of the grooves (12′) at least the groove base is recessed V-shaped and the guide elements (22′) are designed wedge-shaped at least on their outside facing the groove base (12′).
 11. Blown film extrusion device according to claim 1, characterized by the fact that in a section across the radial extent of the grooves at least one V-shaped rib is formed on the groove base and the guide elements are profiled with a wedge-shaped groove at least on their outside facing the groove base.
 12. Blown film extrusion device according to claim 10, characterized by the fact that the enclosed angle in the convex part (22′) is smaller than in the concave part (12′) in a cross section through a linear guide. 